1. Field of the Invention
The present invention relates to a super twisted nematic type simple matrix liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices have been used in a variety of applications as display devices for office automation equipment. Since a high-precision display is required in a display device of this type, the number of pixels is large, and highly time-divisional driving is required. In addition, a high contrast and a large view angle are also required. In order to satisfy these needs, of all the liquid crystal display devices, a super-twisted nematic type (to be referred to as an STN type hereinafter) simple matrix liquid crystal display device capable of performing highly time-divisional driving and exhibiting a relatively high contrast has been used as a display device for the office automation equipment.
This STN type simple matrix liquid crystal device comprises opposite substrates spaced apart from each other by a predetermined distance, electrodes formed on the inner surfaces of the opposite substrates in directions perpendicular to each other, aligning films formed to cover the electrode formation surfaces to align liquid crystal molecules in a predetermined direction, a liquid crystal material sealed between the aligning films, and a pair of polarizing plates arranged outside the pair of substrates so as to sandwich the pair of substrates. Liquid crystal molecules near the aligning films are aligned in a direction of aligning treatment by alignment regulating forces of the aligning films. The molecules are twisted at an angle of 180.degree. to 270.degree. from one substrate to the other substrate along the predetermined direction of aligning treatment by the alignment regulating forces.
The aligning state of the liquid crystal molecules is changed upon application of an electric field across the opposite electrodes. An optical change caused by a change in alignment is visualized by the pair of polarizing plates, thereby performing a desired display.
In order to operate the above liquid crystal display device in accordance with highly time-divisional driving, the twist angle of the liquid crystal material is increased, and to obtain a higher visual contrast, a birefringence effect of the liquid crystal is utilized to undesirably cause coloration of display contents. In addition, since the view angle is small, the display colors change depending on angles at which an observer observes the display.
In order to solve the coloration problem described above, a two-layered STN type liquid crystal device is proposed wherein a drive cell having drive electrodes formed on the opposite substrates and a compensation cell having a twist direction opposite to that of the liquid crystal molecules in the drive cell are stacked to form a two-layered structure.
In this two-layered STN type liquid crystal display device, differences in phase shifts caused by light components having different wavelengths in the drive cell can be compensated by the compensation cell. Coloration of the display contents can be suppressed to obtain an almost black-and-white display.
In the STN type liquid crystal display device described above, since a liquid crystal cell similar to the drive cell is used as the compensation cell, the manufacturing process of the compensation cell is complicated to result in high cost and a bulky display device. Coloring of the display contents cannot be sat satisfactorily eliminated, and the view angle is kept small, resulting in inconvenience. FIG. 1 shows equi-contrast curves in the conventional two layered STN type liquid crystal display device. These equi-contrast curves represent contrast values obtained when viewed from 10.degree., 20.degree., 30.degree., 40.degree., and 50.degree. directions with respect to a normal to the substrates of the liquid crystal display device. A hollow triangle represents a point at which the contrast is 150; a black triangle represents a point which the contrast is 100; a hollow square represents a point at which the contrast is 50; a black square represents a point at which the contrast is 10; and a dot represents a point at which the contrast becomes negative, i.e., the black-and-white display color is reversed. An arrow X represents the direction of the aligning treatment of the aligning film on the light-incident side substrate. As is apparent from FIG. 1, the contrast is very high in the front surface of the liquid crystal display device, i.e., in the direction of the normal to the substrates. The contrast is greatly decreased when the view angle is increased with respect to the direction of the normal. Display color reverse regions appear at the upper left and lower right portions of the liquid crystal display device. I this case, the upper left reverse region is very large in area.
In this conventional liquid crystal display device, when the screen is observed from an upper left position inclined by 40.degree. from the direction of the normal, the screen looks like a negative image, thus resulting in a decisive drawback.
FIGS. 2A, 2B, 2C, and 2D show CIE chromaticity diagrams representing changes in display colors in light-transmitting (ON:.quadrature.) and light-shielding (OFF: *) states when the view angle is sequentially changed from the direction of the normal to the substrates to the 50.degree. direction in units of 10.degree. in an order of upper, left, lower, and right sides of the liquid crystal display device. In this case, an arrow b in each diagram indicates a direction of a change in chromaticity of transmitted light in the ON state, and an arrow a in each diagram indicates a change in chromaticity of transmitted light in the OFF state. As is apparent from FIGS. 2A, 2B, 2C, and 2D, the display colors in the ON and OFF states in the upper, lower, left, and right directions greatly change in accordance with the inclination angles with respect to the direction of the normal to the substrates. This indicates that the display colors are different depending on observation directions with respect to the liquid crystal display device, thus greatly degrading display quality.